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Effects of phosphate intake on distribution of type II Na/Pi cotransporter mRNA in rat kidney
T Ritthaler1, M Traebert, M Lötscher
1Institutes of Anatomy and Physiology, University of Zurich-Irchel, Zurich, Switzerland.
Background:
Renal phosphate (Pi) reabsorption is regulated by dietary Pi intake, as well as in other ways. Changes in Pi reabsorption are associated with the modulation of sodium/Pi cotransporter type II (NaPi-2) protein abundance in the brush border membrane (BBM) of proximal tubules (PTs) and of renal NaPi-2 mRNA levels. In this study, we address whether the NaPi-2 protein and NaPi-2 mRNA distribution patterns in the renal cortex vary in parallel with changes of dietary Pi intake.
Methods:
We investigated in cryosections of perfusion-fixed rat kidneys by in situ hybridization (ISH) and immunohistochemistry (IHC) the distribution patterns of NaPi-2 mRNA and of NaPi-2 protein one week, two hours, and four hours after changes in dietary Pi intake.
Results:
NaPi-2 mRNA and NaPi-2 protein were present in PTs exclusively. In rats adapted to one week of high Pi intake, signals for NaPi-2 mRNA and NaPi-2 protein in cortical PTs were weak, except in the convoluted parts of PTs of juxtamedullary nephrons. After one week of low Pi intake, the ISH and IHC signals for NaPi-2 were high in PT segments in all cortical levels. The switch from a chronic high to a low Pi intake within two and four hours induced no increase and a slight increase, respectively, in the NaPi-2 mRNA signal in PTs of midcortical and of superficial nephrons, whereas in the BBM of these nephrons, NaPi-2 protein was markedly up-regulated. Two and four hours after switching from low to high Pi intake, the overall high ISH signal for NaPi-2 mRNA was unchanged, whereas NaPi-2 protein staining was drastically down-regulated in the BBM of PTs from superficial and midcortical nephrons.
Conclusions:
The marked changes in NaPi-2 protein abundance in the BBM, following altered dietary Pi intake, precede corresponding changes at the RNA level by several hours. Thus, the early adaptation to altered Pi intake involves mRNA-independent mechanisms. The up- or down-regulation of NaPi-2 protein abundance in the BBM and NaPi-2 mRNA in PT affects mainly midcortical and superficial nephrons.
Insights
Dietary phosphate intake rapidly alters sodium/phosphate cotransporter type II (NaPi-2) protein levels in kidney tubules. These protein changes occur hours before corresponding shifts in NaPi-2 mRNA, indicating early, mRNA-independent adaptation mechanisms.
Area of Science:
- Nephrology
- Molecular Biology
- Physiology
Background:
- Renal phosphate reabsorption is crucial for maintaining phosphate homeostasis.
- Sodium/phosphate cotransporter type II (NaPi-2) protein and mRNA levels in proximal tubules (PTs) are key regulators.
- Understanding the dynamic regulation of NaPi-2 is essential for comprehending kidney function.
Purpose of the Study:
- To investigate whether NaPi-2 protein and mRNA distribution patterns in the renal cortex change in parallel with dietary phosphate intake.
- To elucidate the temporal relationship between NaPi-2 protein and mRNA regulation in response to dietary phosphate.
- To determine the role of mRNA-independent mechanisms in early phosphate adaptation.
Main Methods:
- In situ hybridization (ISH) and immunohistochemistry (IHC) were used to analyze NaPi-2 mRNA and protein distribution.
- Cryosections of perfusion-fixed rat kidneys were examined.
- Experiments were conducted at one week, two hours, and four hours after dietary phosphate intake alterations.
Main Results:
- NaPi-2 mRNA and protein were exclusively found in PTs.
- High dietary phosphate intake led to weak NaPi-2 signals, while low intake resulted in high signals.
- Switching to low phosphate intake rapidly upregulated NaPi-2 protein, preceding mRNA changes.
- Switching to high phosphate intake rapidly downregulated NaPi-2 protein, with unchanged mRNA levels.
Conclusions:
- Changes in NaPi-2 protein abundance in the brush border membrane (BBM) precede RNA level changes by several hours.
- Early adaptation to altered dietary phosphate intake involves mRNA-independent mechanisms.
- Regulation of NaPi-2 protein and mRNA primarily affects superficial and midcortical nephrons.